1. Field of the Invention
The present invention relates generally to Global Navigation Satellite System (GNSS) base stations, and in particular to calibrating a network of portable, secondary base or reference stations in relation to a master base station for differential GNSS correction over a coverage region comprising multiple overlapping coverage polygons.
2. Description of the Related Art
Differential GNSS techniques have been successfully applied for a number of years. These techniques, for example, enable accurate real-time positioning of a rover receiver relative to a base receiver. This positioning includes code-only or carrier-smoothed-code differential techniques that result in sub-meter accuracy, such as those employed while operating with older RTCM 104 messages. They include carrier phase based techniques that facilitate centimeter-level real-time kinematic (RTK) positioning.
Differential GNSS (DGNSS), as its name implies, requires that data be differenced. One of the most useful differences in DGNSS, and therefore a widely used difference, is that of differencing two similar observations of satellite ranging signals where one observation is made at a base or reference GNSS receiver and another is made at a rover GNSS receiver. This type of difference, commonly referred to as the single-difference, removes common-mode errors (i.e. errors seen by both base and rover receivers) such as satellite orbit errors, satellite clock errors, and atmospheric errors that arise as the signal passes through the ionosphere and the troposphere. The remaining sources of error that result when employing single-difference techniques are those that are unique to the receiver, such as receiver noise and multipath. These remaining errors are often small in comparison to the common-mode errors, especially when carrier-phase observations are employed. Left unchecked, these errors can, over time, result in relatively large inaccuracies in the differential signals provided by base stations. In DGPS/DGNSS systems, a stationary base receiver uses its known location as a reference for computing differential corrections that correct errors in its own phase observations, and these corrections are then supplied over a communication link to a rover to correct similar errors seen at the rover. Alternatively, the base station supplies its reference location and phase observations over the communication link to the rover for computing the differential corrections itself, or takes a mathematically equivalent approach of forming single-differences between base and rover observations.
It is common practice to obtain differential position information using base stations to provide the additional signals to GNSS satellite positional signals. These base stations typically use either known GNSS position coordinates selected from a list of previously used coordinates or automatically-generated coordinates calculated by averaging GNSS-based position data. However, when a master and multiple secondary base stations are used for covering a large region, one base station's coordinate system may not line up with another base station's system. Thus, when a working vehicle travels from one base station signal coverage area to another, the positions computed by a guidance CPU within the working vehicle may suddenly “jump” when transitioning to the coverage area of the new GNSS base station. The vehicle (e.g., tractor/implement) may be guided along a guide path based on the new base station differential positioning data which does not line up with the previous path, resulting in uneven rows within a field.
To address this problem, Whitehead and McClure U.S. Pat. No. 7,400,294, which is assigned to a common assignee and is incorporated herein by reference, discloses portable reference stations for local differential GPS corrections. A base or reference station location(s) is determined and stored. Either differential correction terms or raw satellite ranges are transmitted to GNSS-equipped remote or rover vehicles for use in differential GNSS (DGNSS) positioning and guidance operations. The base or reference station can be removed and later returned to the general area of its previous location, for which the GNSS position coordinates have been saved. If placed within a predetermined threshold distance of a previously-saved location, the base or reference station will “snap” to the saved reference location and compute GNSS corrector terms using the saved reference location data. Otherwise a new GNSS-based reference position will be computed for immediate use and added to the list of stored reference locations for future use.
What is needed then is a system and method designed to coordinate several base stations together to form one large region of differential guidance based off of a single “known” coordinate for a master base station. Doing this would ensure that when the working vehicle leaves the signal area of one base station and enters into another, the current path that the vehicle is traveling will not deviate or “jump” because of a difference in the “known” coordinates that the base station is basing its differential positional information upon.
DGNSS requires rover and base GNSS receivers. The base is typically stationary at a known location and sends to a rover GNSS receiver phase (or pseudo-range) observations plus its known location, or in lieu of this, differential correctors or other differential enabling data via radios or cell phones. The rover receives the correctors from the base(s) and uses them to correct its own satellite ranging signal observations to increase their accuracy. The result is that the rover can provide a more accurate location using corrected observations, even to the centimeter level or less when carrier phase is used in an RTK solution. The range of a base signal is, however, limited. For large fields or tracts of land, a master and multiple secondary base receivers may be needed to provide differential positioning data to the rovers to cover the entire region.
DGNSS base stations are used to generate and transmit GNSS corrections from base reference positions to rover DGNSS receivers, typically by radio or cellular telecommunication, to improve accuracy to RTK or near RTK (e.g., sub-centimeter) levels. Corrections are sent out from a base system, referenced to the coordinates set for the base, and either manually entered, automatically selected from a list of previously saved points, or automatically selected after a period of data averaging.
Most applications do not need absolute accuracy, but do require high relative accuracy during field operations, both within the same week and for subsequent field operations months or even years later. In agriculture, this allows for planting, cultivating, and harvesting with minimal field disturbance for water and soil conservation, including the ability to drive without damage to sub-surface drip tape used for high efficiency watering.
This invention allows a vehicle to base its entire preplanned path upon the known coordinates of a single master base station, and then transfer those coordinates to calibrate other base stations as it enters the signal area of these secondary base stations, thus expanding the secondary base station network. Essentially, this invention will calibrate secondary base stations to the coordinate information of a single, master base station using a mobile transceiver device within a working vehicle.